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Also known as the ACTH or cosyntropin stimulation test, this medical test is used in the diagnosis of adrenal insufficiency. Adrenal insufficiency leads to a deficiency of corticosteroids, mainly cortisol. This can be a life threatening condition that can present with many vague symptoms, such as fatigue. Since the symptoms are often non-specific, lab tests are essential in making the diagnosis.
Cortisol has widespread effects on metabolism, including helping to maintain a normal blood glucose level. It also affects the immune response and blood pressure. Cortisol is essential to protect the body against stress. Without it, an infection or uncomplicated surgery would lead to a drop in blood pressure, hypotensive shock and death. Cortisol is secreted in response to stimulation by ACTH, or adrenocorticotrophic hormone, which is secreted from the pituitary. Cortisol deficiency can be primary, meaning a problem in the adrenal glands themselves, secondary, due to ACTH deficiency, or occasionally tertiary, caused by a defect in the hypothalamus, which regulates the pituitary.
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When Is A Cortisol Stimulation Test Performed?
Patients who have signs and symptoms of suspected adrenal insufficiency, such as fatigue, low blood pressure and hyperpigmentation, require testing to establish the diagnosis. If the patient is severely ill, for example in shock, treatment comes first. Testing can be done later. Opinions vary as to the exact testing needed. According to Merck.com, the first step in testing is often measuring blood levels of cortisol and ACTH. A low cortisol level accompanied by a high ACTH is diagnostic of adrenal insufficiency. Further testing is required when the levels are not clear cut. According to "Harrison's Principles of Internal Medicine", on the other hand, a cortisol stimulation test is necessary to make the diagnosis of adrenal insufficiency.
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How the test is performed
A baseline cortisol level is drawn from the blood. The patient is then given 250 micrograms of cosyntropin, a synthetic form of ACTH. It can be administered intravenously or by an intramuscular shot. Blood samples for cortisol levels are drawn 30 and 60 minutes later.
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If the adrenal gland is functioning properly, cortisol levels should rise in response to stimulation by ACTH. A level beneath 18 microgram/dl 60 minutes after the stimulation indicates adrenal insufficiency. The next question in diagnosis is what is the cause of the adrenal insufficiency.
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Further Cortisol Stimulation Tests
In order to distinguish between primary and secondary adrenal insufficiency, a more complicated test can be performed. A higher dose of cosyntropin is given initially. Cortisol is then measured at intervals for 24 hours. If the level stops rising after 60 minutes, this means that the adrenals cannot respond to stimulation, and that the cause of insufficiency is in the adrenals themselves. If the level of cortisol continues to rise after 1 hour, the problem lies higher up - in either the pituitary or the hypothalamus.
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1. Harrison's Principles of Internal Medicine, 17th edition; Anthony Fauci wt al.; 2008.
2. Merck.com: Addison's Disease - Adrenal Disorders. http://www.merck.com/mmpe/sec12/ch153/ch153b.html